TOF mass spectrometry is an analytical technique for measuring the mass/charge ratio of ions by accelerating ions and measuring their time of flight to an ion detector.
In a simple form, a TOF mass spectrometer includes an ion source for generating a pulse (or burst) of ions of sample material and an ion detector for detecting ions that have traveled from the ion source to the ion detector. The ions generated by the ion source preferably have, e.g. because they have been accelerated to, a predetermined kinetic energy and so have different speeds according to their mass/charge ratio. Accordingly, as ions travel between the ion source and the ion detector, ions of different mass/charge ratios are separated by their different speeds and so are detected by the ion detector at different times, which allows their respective times of flight to be measured based on an output of the ion detector. In this way, mass spectrum data representative of the mass/charge ratio of ions of sample material can be acquired based on an output of the ion detector.
Matrix-assisted laser desorption/ionization, often referred to as “MALDI”, is an ionisation technique in which, generally, a laser is used to fire light at a (usually crystallised) mixture of sample material and light absorbing matrix so as to ionise the sample material. The sample materials used with MALDI typically include molecules such as biomolecules (e.g. proteins), large organic molecules and/or polymers. The light absorbing matrix is generally used to protect such molecules from being damaged or destroyed by light from the laser. The resulting ions, which typically have masses of several thousand Daltons, are then accelerated to high kinetic energies, typically around 20 keV. Generally, an ion source configured to generate ions by MALDI is referred to as a “MALDI ion source”. A MALDI ion source typically includes a laser for ionising sample material by firing light at a mixture of the sample material and light absorbing matrix.
MALDI is usually combined with time of flight mass spectrometry to provide “MALDI TOF” mass spectrometry in which, generally, a pulse of ions is generated by MALDI and the time of flight of the ions is then measured over distances typically of around 1-2 meters so that the mass/charge ratio of the ions can be determined.
Measuring the time of flight of ions in modern TOF mass spectrometers, e.g. MALDI TOF mass spectrometers, typically requires a diverse range of high speed digital and analogue electronics. For example, high speed timing electronics may be used in order to accurately synchronise various high-voltage electrical pulses with the firing of a laser and the acquisition of an ion signal. Also, kV/μs slew-rate high voltage electrical pulses may be used to accelerate, gate and steer ionised molecules generated by the laser. Finally, high speed multi-bit analogue to digital converters may be used to record the output from an ion detector so that the time of flight of the ions, and therefore the mass/charge ratio of the ions, can be determined. Such high speed digital and analogue electronics are typically run for each acquisition cycle of the TOF mass spectrometer.
Until recently, TOF mass spectrometers, e.g. MALDI TOF mass spectrometers, have used gas lasers having a repetition rate (rate at which it can fire pulses of light) of up to a few tens of Hz. More recent TOF mass spectrometers have used solid-state lasers capable of much higher repetition rates, e.g. 1 kHz or more.
Generally, when a MALDI ion source is in use, a laser of the MALDI ion source fires a pulse of (e.g. UV) light at a mixture of sample material and light absorbing matrix contained in a sample spot so as to eject a plume of ionised and non-ionised (i.e. neutral) sample material (“analyte”) and light absorbing matrix from the sample spot. The ionised material contained in this plume (mostly ions of sample material and some ions of light absorbing matrix) will generally be accelerated away by an electric field produced by electrodes of the MALDI ion source so as to pass through apertures in the electrodes, e.g. for subsequent detection by an ion detector. However, the non-ionised material contained in this plume (mostly non-ionised light absorbing matrix and some non-ionised sample material) will generally continue to expand from the sample spot until it is deposited on surfaces in the vicinity of the ion source, e.g. surfaces of the electrodes of the MALDI ion source.
Over time, the non-ionised material builds up on the surfaces in the vicinity of the sample spot, particularly on the surfaces of the electrodes of the MALDI ion source, to form an insulating layer of contaminant material that may charge up over time and adversely affect the operation of the MALDI ion source. In particular, the insulating layer of contaminant material on the electrodes can distort the electric field produced by the electrodes such that the sensitivity or resolution of a mass spectrometer using the MALDI ion source is degraded. At this point the electrodes of the MALDI ion source will generally require cleaning.
For many years the principal method of cleaning the electrodes of a MALDI ion source was to vent and open an evacuated housing containing the electrodes to allow the electrodes to be cleaned in situ or removed completely for thorough cleaning. In both cases, in addition to the cleaning time, several hours were generally required to restore a vacuum to the housing of the MALDI ion source (once closed) and to perform high voltage conditioning, instrument tuning and mass calibration procedures that are generally necessary for the MALDI ion source to be used in mass spectrometry.
In many applications (e.g. biochemistry) there is a growing requirement for higher throughput mass spectrometers, which can now be realised by the introduction of MALDI ion sources capable of running at repetition rates of 1 kHz or over. This has increased the rate of contamination build up on the electrodes of MALDI ion sources, and the frequency with which they must be cleaned, to such an extent that it is generally no longer practical to vent the MALDI ion source every time its electrodes require cleaning.
These considerations make it desirable to find an effective method to clean the electrodes of MALDI ion sources without requiring an evacuated housing of the MALDI ion source to be vented.
Various methods have been considered to clean the electrodes of MALDI ion sources without the need to vent an evacuated housing of the MALDI ion source.
For example, in GB2398923, Holle and Franzen proposed a method which uses a specially designed cleaning plate that is inserted into a MALDI ion source in place of a standard sample plate to clean a first electrode by spray-washing with solvent or mechanically with cleaning scrubbers.
In U.S. Pat. No. 7,541,597, Holle and Przybyla propose a method of cleaning electrodes of MALDI ion sources by etching with reactive ions produced by an electrically generated gas discharge in a specially admitted reactant gas, which can be automatically carried out by using a specially designed electrode place in place of a standard sample plate carrier and admitting a reactant gas.
The above mentioned methods share a disadvantage in that a special apparatus has to be inserted in place of a standard sample plate such that the precise location of sample material may be lost, which may be important in certain imaging applications. A further disadvantage may be the interruption of automated runs for mass spectrometers capable of automatically loading several sample plates.
Methods of cleaning the electrodes of MALDI ion sources have also been proposed in which the electrodes are heated to temperatures of up to 250° C., e.g. using contact heaters (U.S. Pat. No. 6,953,928, Vestel et al.) or with infrared laser radiation (GB2457362, Holle and Hohndorf). The effectiveness of heating the electrodes of MALDI ion sources has been found to be variable and depends very much on the light absorbing matrix used. For example, whilst DHB (2,5-dihydroxybenzoic acid) has been found to be readily removed by heating to temperatures of around 150° C., CHCA (α-Cyano-4-hydroxycinnamic) has been found to be much more stubborn and difficult to remove, even when heated to over 200° C. The amount of contaminant material present has also been found to have a significant effect on the effectiveness of heating in that it has been found much easier to remove thin layers of contaminant material by heating compared with relatively thick layers that can build up even in a relatively short time. Further, some contaminant materials, particularly polymers, can be very difficult to remove simply by heating.
The present invention has been devised in light of the above considerations.